Sulfonated poly(ether ether ketone) membranes have greater hydrophilicity relative to membranes prepared from the parent polymer, but are subject to excessive swelling and dimensional instability when wetted. In the context of preparing membranes for use as separators in batteries and fuel cells crosslinking of sulfonated poly(ether ether ketone) has been pursued as a means of overcoming this deficiency. The publication of Hou et al (2012) reviews methods of crosslinking of sulfonated aromatic polymers that have been pursued in the context of developing PEMs. Many of these methods exploit the reactivity provided by the presence of the sulfonic acid group.
The publications of Mikhailenko et al (2004), Mikhailenko et al (2006), Deb et al (2007) and Hande et al (2008) disclose methods where a condensation reaction between the sulfonic acid of the sulfonated aromatic polymer and the hydroxyl group of an alcohol forms a sulfonic acid ester linkage. These methods effectively lower the degree of sulfonation (DS) of the substrate polymer. The publications of Mikhailenko et al (2004 and 2006) disclose polymer electrolyte membranes prepared from sulfonated poly(ether ether ketone) and simple polyols. Structural data indicated that under the conditions used simple polyols such as ethylene glycol and glycerol do not link the neighbouring main chains of the polymer via the sulfonic acid functions, but form an interpenetrating network of oligomers bonded to the sulfonyl (SO3) group. As a major proportion of the sulfonic acid functions are not involved in this form of “cross-linking”, membrane conductivities are only somewhat reduced.
The publication of Rhoden et al (2011) discloses a method of cross-linking a sulfonated poly(ether ether ketone) with a high degree of sulfonation using 1,4-benzene dimethanol. The method is a zinc chloride catalysed crosslinking that is stated to produce a high hydrophobic polymer backbone, whilst still maintaining high levels of polymer sulfonation. The method is distinguished from those disclosed in the publications of Mikhailenko et al (2004), Mikhailenko et al (2006), Deb et al (2007) and Hande et al (2008) in that the sulfonic acid group does not participate directly in the reaction resulting in the formation of the cross-link. The reaction is performed in an aqueous solution of SPEEK.
The publication of Di Vona et al (2008) discloses sulfonation in the preparation of proton-conducting hybrid polymers based on poly(ether ether ketone). The use of both sulfuric acid and chlorosulfonic acid as the sulfonating agent is disclosed.
The publication of Hande et al (2008) discloses the crosslinking of sulfonated poly(ether ether ketone) in the preparation of proton exchange membranes using 2,6-bis(hydroxymethyl)-4-methyl phenol and 1,4-bis(hydroxymethyl) benzene as the cross-linking agents. The crosslinking reaction was achieved by a thermally activated condensation reaction between the hydroxymethyl group of the cross-linking agent and the sulfonic acid group of the polymer.
The publication of Di Vona et al (2009) discloses thermally induced crosslinking of sulfonated poly(ether ether ketone) by the formation of SO2 bridges between macromolecular chains. The publication also discloses the important role played by the casting solvent when seeking to improve the proton-exchange membranes used in fuel cells.
The publication of Ye et al (2009) discloses the use of benzoxazine or sulfonic acid containing benzoxazine as a crosslinking agent in the preparation of sulfonated poly(ether ether ketone) proton exchange membranes. The cast membranes are heated at 180° C. for three hours to complete the crosslinking.
The publication of Merle et al (2014) also discloses the preparation of crosslinked sulfonated poly(ether ether ketone) membranes where crosslinking was performed via the Friedel-Crafts route employing 1,4-benezene dimethanol as the cross-linking agent. High proton conductive membranes were obtained at increased temperatures.
In the context of preparing membranes for use as separators in batteries and fuel cells the objective is to prepare a membrane that permits the passage of ions whilst maintaining the separation of the two electrodes. By contrast, in the context of preparing membranes for use in processes driven by hydrostatic or osmotic pressure, the objective is to prepare a membrane that permits the passage of water whilst limiting the passage of solutes. In food processing the membrane is also required to be chemically resistant and durable.
Osmosis is generally seen as the movement of water from a solution of higher water chemical potential to one of lower water chemical potential. This movement, or flux, is moderated by a semi-permeable membrane, which allows the passage of water, but not the passage of the species whose presence lowers the chemical potential of water in the receiving solution. This fundamental thermodynamic property of solutions is an essential component of many biological processes (McCutcheon and Wang (2013)).
The first viable semi-permeable membrane was made in the 1960s from cellulose acetate and used in reverse osmosis (Loeb (1981)). The further development of thin film composite membranes followed with the introduction of the concept of interfacial polymerisation (Mogan (1965)). In a thin film composite membrane, each individual layer can be optimised for its particular function. The thin “barrier layer” can be optimised for the desired combination of solvent flux and solute rejection, while the porous “support layer” can be optimised for maximum strength and compression resistance combined with minimum resistance to permeate flow. Numerous reviews concerning the preparation and properties of composite membranes developed for use in reverse osmosis and nanofiltration are available (e.g. Petersen (1993)).
The desired properties of membranes used in water desalination, purification or recovery include high rejection of undesirable species, high filtration rate and good mechanical strength. Depending on the particular application on which the membrane is used other desired properties may also include resistance to fouling and chemical decomposition (McCutcheon and Wang (2013)). These latter properties are particularly desirable for membranes used in food processing applications such as dairy processing operations where periodic in situ cleaning and sterilisation of the membrane is required.
It is an object of the present invention to provide at least one method of preparing cross-linked poly(ether ether ketone) suitable for use as the rejection layer of an asymmetric composite membrane. It is an object of the present invention to provide an asymmetric composite membrane suitable for use in the recovery of water from dairy feed streams. It is an object of the present invention to provide a durable asymmetric composite membrane. It is an object of the present invention at least to provide a useful choice in the selection of an asymmetric composite membrane. These objects are to be read in the alternative.